Sign the hash in the the PKCS1_PSS doctest, not the key

As it stood before this commit, the hash was never used in the signing 
process.  It looks like the bug was introduced by e053629 (Restructure both
PKCS#1 signature schemes as objects, 2011-10-16), which changed:

    -    >>> signature = PKCS1_PSS.sign(h, key)
    +    >>> signer = PKCS1_PSS.new(key)
    +    >>> signature = PKCS1_PSS.sign(key)